1. Field of the Invention
The present invention relates to a magnetostriction type stress detector to be used for measurement control of a robot motor having a drive shaft or an automotive engine.
2. Discussion of Background
FIG. 2 shows a construction of a conventional magnetostriction type stress detector as described in Japanese Unexamined Patent Publication No. 211030/1982, for example. Referring to FIG. 2, a reference numeral 1 designates a passive shaft to which a torque is applied, and a reference numeral 2 designates a pair of magnetic layers fixed to an outer circumference of the passive shaft 1. Each of the magnetic layers 2 is formed of a magnetic material having a high magnetic permeability and a soft magnetism, and it is constructed of a plurality of strip-like elements. The two magnetic layers 2 are arranged symmetrically with each other, and are inclined at .+-.45.degree.. A numeral 3 designates a pair of detection coils provided around the magnetic layer 2.
In operation, when a torque is applied to the passive shaft 1, a stress is generated in the magnetic layers 2 with respect to a principal axis of strain in a longitudinal direction thereof. This stress operates as an extension force in one of the magnetic layers 2, while operating as a compression force in the other. Accordingly, there is generated a change in magnetic permeability between the two magnetic layers 2. In the case that a constant of magnetostriction is positive, the magnetic permeability is increased when the extension force is applied, while being decreased when the compression force is applied. In the case that the constant of magnetostriction is negative, the above relationship is reversed. The detection coils 3 serve to generate a magnetic flux which penetrate into the magnetic layers 2 to detect a change in magnetic permeability of the magnetic layers 2 as a change in magnetic impedance, thereby detecting the stress. As the outputs from the detection coils 3 are different in polarity from each other, a differential value of the outputs is obtained as a large output.
However, there is a large difference in the coefficient of linear expansion between the passive shaft 1 and the magnetic layers 2 which causes the generation of a thermal stress in the magnetic layers 2. The thermal stress overlaps the stress to be measured, so that precise measurement of the stress cannot be conducted. To solve this problem, it has been proposed that the passive shaft is formed of a magnetic material having a high magnetic permeability and a soft magnetism, and a magnetic shielding layer for shielding the penetration of a magnetic flux by its magnetic skin effect is selectively formed on the passive shaft, while the magnetic layers are formed on a portion of the passive shaft where the magnetic shielding layer is not formed. In this conventional proposed technique, as the passive shaft and the magnetic layers are formed of the same material, the generation of a thermal stress is prevented to thereby conduct precise measurement of the stress.
However, in the above-mentioned prior art stress detector including the passive shaft and the magnetic layers formed of the same material, much time and labor are required for the selective formation of the magnetic shielding layer on the passive shaft, causing an increase in cost and no possibility of mass production. Furthermore, as the magnetic shielding layer is fixed to the passive shaft, there is a possibility of a thermal stress being generated due to a difference in the coefficient of linear expansion between the magnetic shielding layer and the passive shaft, resulting in a detection error.